1. Field of the Invention
The present invention relates generally to the field of aerogels, and more particularly to a method of preparing aerogel thin films at ambient pressure.
Aerogels are unique solids with up to 99% porosity. Such large porosities confer a number of useful properties on aerogels including high surface area (often exceeding 1000 m.sup.2 /g), low refractive index (n&lt;1.1), low dielectric constant, low thermal-loss coefficient (&lt;0.5 W/(m.sup.2 K), and low sound velocity (100 m/s). These properties lead in turn to potential thin-film applications such as: ultra-low dielectric constant interlayer dielectrics, optically reflective and antireflective coatings, flat panel displays, sensors, superinsulated architectural glazing, catalyst surfaces, and acoustic impedance matching devices. To date, however, the potential of aerogels has not been realized in these applications, because conventional supercritical aerogel processing is energy intensive, often dangerous, and, most important, not amenable to continuous or semi-continuous thin-film forming operations such as dip-coating.
With thermal conductivities as low as 0.02 W/mK, some of the more promising potential applications of aerogel films are based on their insulating properties. Radiative-diode systems which effectively transmit solar radiation but prevent thermal leakage, energy-efficient greenhouses, translucent insulation for solar collectors, and thermally efficient architectural glazing are some of the possibilities. The use of aerogels in such applications is especially attractive due to environmental concerns over conventional insulating materials. Since the original synthesis of silica aerogel in 1931, aerogels have been prepared from both inorganic and organic precursor gels over wide compositional ranges. The conventional means of aerogel synthesis entails placing a liquid-filled gel in an autoclave and increasing the temperature and pressure until the critical temperature and pressure of the pore liquid are exceeded (for ethanol T.sub.c =243.degree. C. and P.sub.c =63 bar). The supercritical pore fluid is then removed from the gel while the temperature is maintained above critical. It is desired that the pore fluid be removed under supercritical conditions where there are no liquid/vapor interfaces. In this case the capillary pressure P.sub.c developed in the liquid equals zero: EQU P.sub.c =-2.sub..gamma.LV cos.theta./r.sub.p= 0 (1)
where .sub..gamma.LV is the liquid-vapor surface tension, .theta. is the wetting angle, and r.sub.p is the pore radius. Drying occurs often with little or no shrinkage, essentially preserving the wet gel structure. An alternate low-temperature method involves replacing the original pore fluid with liquid CO.sub.2 and then removing CO.sub.2 above its critical point (T.sub.c =31.degree. C., P.sub.c =73 bar).
Although present aerogels exhibit unique properties, they suffer several drawbacks for widespread commercial applications, viz.: 1) High pressures (and often, temperatures) required for supercritical processing result in high processing and capital equipment costs and lead to safety and health concerns. 2) For the high-temperature extraction process, significant chemical and physical changes occur during drying that degrade properties (e.g., coarsening of microstructure), whereas the low-temperature process limits the choice of pore fluids to those miscible with CO.sub.2. 3) Aerogels prepared by present processing techniques are hydrophilic and often reactive (alkoxylated surfaces). Condensation of moisture in the pores subjects the gel to capillary stresses that ultimately degrade the structure and the associated insulating properties. 4) Supercritical drying is normally a batch operation performed within the constraints of an autoclave. This precludes continuous forming operations such as thin-film coating and fiber drawing and limits the size of the article to the size of the autoclave.
When drying a gel by evaporation of the pore fluid, the curvature of liquid-vapor menisci developed at the drying surfaces causes the liquid to be in tension. The magnitude of the tension is expressed by the Laplace equation (Eq. 1). This tension is supported by the solid phase causing it to shrink. Shrinkage stops when the tension in the liquid is balanced by the network modulus K.sub.p, which increases during shrinkage as a power law, K.sub.p =K.sub.o (V.sub.o /V).sup.m, where K.sub.o is the initial bulk modulus of the gel, V.sub.o and Vare the initial and current gel volumes, respectively, and m=3.0-3.8. At the critical point where shrinkage stops, the shrinkage of the network .epsilon..sub.v attributable to drying is: ##EQU1## where .phi.s is the volume fraction solids. Strategies to minimize drying shrinkage therefore include increasing the modulus, decreasing the surface tension, and increasing the wetting angle. Condensation reactions that accompany shrinkage normally cause the drying shrinkage to be irreversible, i.e., as hydroxyl-terminated surfaces are brought in contact, they undergo dehydration to produce M-O-M bonds. These covalent bonds preserve the shrunken state of the gel after complete removal of the pore fluid and elimination of the capillary pressure. An additional strategy to minimize shrinkage therefore is to render the gel surfaces unreactive towards condensation, allowing any drying shrinkage to be reversible.